In the semiconductor industry, there is a continuing trend toward higher device densities. To achieve these high densities there has been and continues to be efforts toward scaling down the device dimensions on semiconductor wafers. In order to accomplish such high device packing density, smaller and smaller feature sizes are required. Since numerous conductive features are typically present on a semiconductor wafer, the trend toward higher device densities is a notable concern.
The requirement of small features (and close spacing between adjacent features) requires high resolution lithographic processes. In general, lithography refers to processes for pattern transfer between various media. It is a technique used for integrated circuit fabrication in which a silicon slice, the wafer, is coated uniformly with a radiation-sensitive film, the photoresist. The photoresist coated substrate is baked to evaporate any solvent in the photoresist composition and to fix the photoresist coating onto the substrate. The baked coated surface of the substrate is next subjected to selective radiation using a mask; that is, a mask is employed to effect an image-wise exposure to radiation. The mask permits radiation to contact certain areas of the photoresist and prevents radiation from contacting other areas of the photoresist. This selective radiation exposure causes a chemical transformation in the exposed areas of the photoresist coated surface. Types of radiation commonly used in microlithographic processes include visible light, ultraviolet (UV) light and electron beam radiant energy. After selective exposure, the photoresist coated substrate is treated with a developer solution to dissolve and remove either the radiation-exposed or the unexposed areas of the photoresist (depending upon whether a positive photoresist or a negative photoresist is utilized) resulting in a patterned or developed photoresist.
The mask is a critical element in lithography. Defects in the mask lead to imprecise exposure and consequent decreases in resolution, precise pattern formation and/or the quality of subsequent processing steps. For example, a contaminant particle on a mask may prevent radiation from contacting an area of a photoresist that should receive radiation, resulting in an incompletely exposed photoresist, which would lead to an undesirable pattern formation in the subsequently developed photoresist. Mal-formed structures inhibit the proper function of semiconductor devices.
Contaminant particles that adhere to a mask surface and are capable of at least partially blocking light that should pass through the mask constitute mask defects. Moreover, airborne contaminant particles that are capable of at least partially blocking light that should pass through the mask constitute defects as well. The concern or potential damage attributable to contaminant particles increases as the wavelength of light used to selectively expose photoresists decreases. That is, it is more likely for a contaminant particle to block light having a small wavelength compared to light having a large wavelength. This is especially true for small wavelengths below about 160 nm. Small wavelengths below about 160 nm involves next generation lithography (NGL). The concern over contaminant particles is great because one defect or particle on a mask may constitute a fatal defect requiring expensive and burdensome mask replacement. Moreover, as geometries shrink, small defects have an increasingly detrimental impact on NGL processing.
A pellicle is a transparent, thin membrane (often 99% or more light transmission) that seals off the mask surface from airborne particles and is not damaged by repeated illumination over time. Pellicles are therefore useful for preventing contaminant particles to cause defects in photolithography. However, at small wavelengths below about 160 nm, suitable pellicles with such properties (good light transmission, no damage due to illumination exposure over time) are unavailable.
When a photoresist clad semiconductor substrate is charged into a processing chamber, such as a reticle chamber, it is desirable for the lithography mask to be free of contaminant particles. However, it is difficult to remove contaminant particles from a mask, as it is often required to remove the contaminated mask from the processing chamber. It is also difficult to remove contaminant particles from a mask without damaging the mask or other hardware within the processing chamber. Since NGL masks are expensive, since suitable NGL pellicles are unavailable, and since NGL masks are particularly difficult to repair, it is highly desirable to avoid any damage to NGL masks.